1. Field of the Invention
The invention relates to integrated optics, more particularly, to switching and redirecting of light beams in optical waveguides. Manipulation of waveguide modes, which are light beams inside integrated optical devices, is a powerful capability that has useful applications primarily but not exclusively in displays and telecommunications, such as distribution of light in flat panel displays, re-routing of Wavelength-Division Multiplexed communications channels, and other integrated optics switching tasks.
The following documents are incorporated herein by reference:
1) U.S. Pat. No. 5,544,268, August 1996, Bischel el al, xe2x80x9cDisplay panel with electrically controlled waveguide-routingxe2x80x9d
2) U.S. Pat. No. 3,801,185, April 1974, Ramaswamy et al, xe2x80x9cSwitch for thin-film opticsxe2x80x9d
3) U.S. Pat. No. 5,009,483, April 1991, and U.S. Pat. No. 5,106,181, April 1992, Rockwell III, xe2x80x9cOptical waveguide display systemxe2x80x9d
4) J. Viitanen and J. Lekkala, xe2x80x9cFiber optic liquid crystal displaysxe2x80x9d, SPIE Vol. 1976, High Definition Video, pg. 293-302 (1993)
5) U.S. Pat. No. 5,045,847, September 1991, Tarui et al, xe2x80x9cFlat display panelxe2x80x9d
6) R. Akkari et al., xe2x80x9cThermo-optic mode extinction modulation in polymeric waveguide structuresxe2x80x9d, Journ. Non-Cryst. Solids, 187, pg. 494-497 (1995) p07) U.S. Pat. No. 4,648,687, Yoshida.
Whereas in conventional optics, light beams are switched and manipulated by placing discrete optical elements such as mirrors, lenses, prisms, and etalons in their path, in integrated optics these functions are performed much more compactly, inside solid material structures that contain both optical waveguides and the means for manipulating the light beams traveling therein. The term xe2x80x9clightxe2x80x9d as used herein denotes xe2x80x9celectromagnetic energyxe2x80x9d and xe2x80x9coptical energyxe2x80x9d or xe2x80x9coptical powerxe2x80x9d in general without restriction to visible wavelengths. It is known in the art as a basic principle that refractive index differences and gradients are key to guiding of light waves as well as to their manipulation, in solid materials. An optical waveguide, well known in the art, is an elongated region of material with a higher index of refraction called a core, typically surrounded by material with a lower index of refraction called a cladding, both materials being optically transparent to a greater or lesser degree. The core is laterally narrow but elongated along the desired path of light travel. Light traveling within such a structure becomes concentrated or confined to the higher index region, in a spatial intensity distribution that is called an optical waveguide mode. It is understood in the art that, as a guided light beam encounters different magnitudes of the refractive index arranged in different geometric shapes in the material, its mode shape changes in spatial extent, local intensity, and direction of travel accordingly. In known devices the electro-optic effect with electrical actuation, and the thermo-optic effect with actuation by an electrical heater element or by heating due to optical absorption of light emitting diode or diode laser light of suitable wavelength, have been employed to produce refractive index changes in response to an applied control signal.
Several techniques are known and have been used in the prior art of integrated optics for switching and redirecting light beams in optical waveguides.
Waveguide switches called TIR (Total Internal Reflection) or PIR (Partial Internal Reflection) switches, for routing of light beams to illuminate pixels in a display, are described in Bischel U.S. Pat. No. 5,544,268. The switches are based on electro-optically, thermo-optically, acousto-optically or magneto-optically creating a lower refractive index discontinuity situated at an angle to the waveguide direction so as to cause total internal reflection or partial internal reflection of the guided beam through a significant angle, upon application of a voltage or heat.
Waveguide switches variously called cut-off modulators, mode-extinction modulators, or cut-off switches, are described in other references. These devices are based on decreasing the refractive index difference between core and cladding of a section of optical waveguide below a value required for local confinement and guiding of light, at the wavelengths of interest in an application, by means of the electro-optic, thermo-optic or acousto-optic effect. If the refractive index difference is decreased below this value, all wavelengths become unguided (guiding becomes xe2x80x9ccut offxe2x80x9d), as the material is then substantially uniform with no local confinement of light. In terms of electromagnetic field theory, guided wave propagation is cut off and the light beam spreads out by diffraction, at a rate determined by the guided beam diameter entering the cut-off region, smaller modes generally diverging faster.
Most of the prior art switches and modulators suffer from several shortcomings including high drive power requirement, large size on an integrated optics chip, and inability to control the angle at which light is coupled out in the waveguide cut-off state. The present invention overcomes these problems by employing a different type of refractive index change, thereby providing a much smaller and more efficiently actuated device with more control over the out-coupling angle.
The invention provides an anti-waveguide routing structure, also called a waveguide switch or mode manipulator, that is a section of an otherwise permanent optical waveguide with a controllable refractive index change which exceeds that required to merely suppress waveguiding, thereby forcing a sharp redirection of the light beam from its original direction into selected transverse directions.
According to the invention, roughly described, an optical switch has a first state and a second state. In the first state, a structure in the switch confines one or more optical modes to propagate along a first, unswitched path. The switch is switched into the second state by reducing the refractive index along the first path, or alternatively increasing the refractive index of a region of the switch outside but adjacent to the first path, until the index within the first path is lower, preferably substantially lower, than that of the adjacent region. This creates an anti-waveguiding section in which light is forced to diverge from the unswitched path within a short distance, and into a desired transverse direction, by a combination of diffraction, owing to removal of waveguiding confinement; and refraction, owing to bending of the wavefront toward a higher-index region, and in some embodiments reflection from a high-low index boundary surface. The refractive index change is produced by known means such as the thermo-optic effect, the electro-optic effect, or other suitable effect such as magneto-optic or acousto-optic, together with its associated actuation means known in the art such as a thin-film electrically powered element for heating, or heating provision by a light beam, or electrodes for applying an electric field.
Particular selected transverse directions of weak confinement lead to planar, asymmetrical planar, and vertical versions of the anti-waveguide routing structure. The planar version has weak confinement laterally in the horizontal plane, equally on both sides of the confinement region, and upon actuation, light is forced out of the confinement region into two beams that are deflected symmetrically in lateral directions while remaining vertically confined. The asymmetrical planar version may have weak confinement only on one lateral side of the confinement region, and upon actuation, light from the confinement region is forced preferentially into a second path in the horizontal plane, while remaining vertically confined. Asymmetrical planar operation can also be realized in a structure with horizontally symmetrical weak confinement but an asymmetrically positioned actuation means such that, upon actuation, an anti-waveguide region is created primarily to one side of the center line of the waveguide. A vertical embodiment has weak confinement at a lower cladding layer located on the opposite side of the core from the actuation means, but stronger confinement in the other transverse directions, such that upon actuation, light from the confinement region is forced into a second path that diverges away from the actuation means, and generally in the downward direction.